Method of manufacturing a distributed light emitting diode flat-screen display for use in televisions

ABSTRACT

A novel display screen structure and method of manufacturing such screens for use, for example, in large screen television displays. The process of the present invention is one which can be accomplished with no new materials, no critical geometric requirements such as critical separations and alignments and only low voltage drivers. The combination of these features results in a technology which can be easily scaled to large sizes to provide relatively low-cost large screens for televisions. An important step in a first embodiment of the present invention is the alignment of a large plurality of columnar-shaped light emitting diode slivers in an uncured optical epoxy by applying an electric field through a mixture of such slivers and epoxy and then curing the epoxy to effectively fix the light emitting diode slivers in that aligned configuration. In a second embodiment, the LED slivers are mixed with molten glass which is formed into elongated glass fibers. The fibers are sandwiched between conductive glass plates and heated while an aligning voltage is applied. The light being emitted by such diodes is thereafter controlled by orthogonally directed electrodes at least one of which is optically transparent and which are placed on opposing surfaces of the thin plate-like structure fabricated in accordance with the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.07/609,404 filed on Nov. 5, 1990 now U.S. Pat. No. 5,102,824, issuedApr. 7, 1992.

TECHNICAL FIELD

The present invention relates generally to the manufacture of largescreen displays, such as those used in flat-screen televisions, and morespecifically, to a manufacturing technology for making large screendisplays, wherein light emitting diode wafers are divided anddistributed across the screen.

BACKGROUND ART

Large screen televisions have not made deep in-roads into the consumermarket for a number of reasons. Currently, the largest cathode ray tubesare approximately thirty-five inches measured diagonally and are veryheavy, i.e. over 100 pounds. Vacuum technology limits the size of thescreen. The larger the screen, the more glass is needed to keep thevacuum intact and the heavier the screen gets. Direct view tubetechnology, after serving as the work-horse of the industry, has reachedits limits when it comes to larger screens. The big screen televisionscurrently being marketed are of the projection type. These screens use asmall, extremely bright source (either a cathode ray tube or atransmissive liquid crystal display screen) and magnify it to a largesize using conventional optics. Besides being bulky, such screens have anumber of practical limits. There exist trade-offs between sourcebrightness, tube lifetime, optical aberrations and viewing angle thatgenerally result in poor image quality for such systems, as compared todirect view cathode ray tube televisions.

In essence, what one would like is a screen technology that scales upbetter with size. Twenty five inch tubes cost orders of magnitude lessto produce than a forty inch tube which has three times the screen areaof a conventional size tube. Tube technology does not scale well, andthus what one needs is a screen that costs a fixed amount per squarefoot. In addition, although most televisions today are sold as largeboxes, one would expect large TVs of the future to be flat screens,simply because a six foot screen contained in a six foot square box isnot very practical, but a six foot screen, mounted on a wall and takingup only wall space, would be practical.

A number of flat panel technologies intended for large screen home usehave been explored in the last twenty years or so, but each has its ownproblems. Liquid crystal displays are most often seen in calculators andlap-top computers. The liquid crystal is sandwiched between twopolarizers and introduces a voltage controlled polorization rotation ofthe incident light. Thus, elements of the liquid crystal display can bemade clear or opaque, simply by applying a voltage. The problem withthis technology is that it does not scale very well. The amount of phaserotation, in addition to being proportional to the applied voltage, isalso proportional to the distance between the front and back polarizers.This distance must be controlled to a few tens of microns across theentire area of the screen. Unfortunately this is virtually impossiblebecause large screens are simply not sufficiently rigid, i.e. they flexunder gravity. Making such screens thicker is not a viable solutionbecause thicker screens get even heavier.

The LCD is a voltage controlled device. In order to selectively turn"on" one pixel, a voltage, V_(x), must be applied to a horizontalelectrode (corresponding to that pixel) and another voltage, -V_(y),must be applied to the vertical electrode, so that the pixel sees avoltage drop of V_(x) +V_(y). Note that if all other horizontal andvertical electrodes are held at ground potential, the screen will haveother pixels with V_(x) and V_(y) across them. Thus, with a voltagecontrolled device, "crosstalk" occurs, i.e., turning "on" one pixelslightly turns "on" other neighboring pixels.

The LCD industry has gotten around this by making an "active matrix" LCDwhere each pixel has its own transistor driver. This resolves thecrosstalk problem but introduces severe manufacturing constraints. Chiplithography (accurate to ˜1 μm alignment) across a large screen ( ˜1m²)with multiple mask layers is nearly impossible and not very costeffective since the yield is very low. Ideally, what one wants is adirectional current device (a diode) at each pixel to eliminatecrosstalk, no critical alignments, and robustness against pixelfabrication errors. This fact has limited liquid crystal display screensto small sizes, such as less than thirteen inches diagonally.

Another flat panel technology that suffers from this same problem isplasma display technology. In this technology, exciting electrodes mustbe properly spaced across a large area. This is not a trivial task. Inaddition, plasma displays require high driving voltages, i.e. a fewkilivolts. The nature of flat panel technology requires X--Y addressingelectrodes, one for each horizontal and vertical line of resolution.Thus a thousand by thousand pixel screen requires two thousand drivelines. Plasma displays need high voltage drivers (i.e., high voltagesemiconductors) for every line, making them prohibitively expensive forhome use.

Another major flat panel technology consists of electroluminescentscreens made of phosphors, sandwiched between X--Y electrodes. Thisdevice is solid and does not have plate separation problems like thepreviously discussed approaches. It does, however, require high voltagedrivers to excite the phosphors, thereby again making high resolutioninfeasible for home use.

There is therefore an ongoing need to provide a way of overcoming theobstacles of the noted previous technologies to provide an improveddisplay screen and method of manufacture thereof, which is especiallyconducive to the manufacture of large screen televisions withoutsuffering the aforementioned disadvantages of the prior art.

SUMMARY OF THE INVENTION

The present invention comprises a novel method and apparatus exploitinglight emitting diodes to overcome the obstacles of previoustechnologies, including such obstacles as large area plate separationtolerances and high voltage line drivers. The basic properties ofstandard light emitting diode technology match the needs of displaytechnology. Light emitting diodes are reasonably efficient in convertingelectrical power to optical power and require only low driving voltages.However, standard light emitting diodes are much too bright and requiretoo much current for use in everyday television screens. Furthermore,one of the most critical needs in manufacturing technology fortelevisions is that it be simple and thus low cost. Therefore, placing apre-packaged light emitting diode at every pixel location in a TVdisplay would not be easy, nor inexpensive. Idealy, what would bedesirable is a "spray-on" technology, where the screen is made bycoating a substrate with something easily applied and then addingelectrodes. The present invention affords a unique low costmanufacturing technique which is tantamount to such a "spray-on"technology.

The first step in the method of the present invention comprises thegrowth of a wafer with a light emitting PN junction and a sacrificialbuffer layer such as of aluminum arsenide. Contact layers are grown onboth sides of the junction and a metal contact such as a gold germaniumcontact is alloyed at this stage. Eventually, light emitting diodeslivers are produced and these slivers are mixed with a curable opticalepoxy. The light emitting diode slivers are then aligned electricallybefore the epoxy is cured. Ultimately, horizontal and verticalelectrodes are applied using lithographic techniques, producing a largearray of light emitting diode slivers whose pixels are easilyaddressable with low voltage drivers connected to horizontal andvertical lines. This technology can be readily used for making colortelevision screens by combining three separate screen havingrespectively red, green and blue light emitting diode slivers. In asecond embodiment of the invention, the LED slivers are mixed withmolten glass and formed into a cylinder. The cylinder is then drawn intoa glass fiber. Three different color fibers are made, one for each ofred, green and blue. These fibers are then sliced into lengths equal tothe screen width and placed between two alignment plates. The plates areheated and a voltage is applied to align the LEDs. The glass fibers flowtogether to form a continuous plate of material. The electrodes are thenapplied using the described lithographic techniques. The method of thepresent invention may thus be used for producing large screen televisiondisplays which involve no new materials, no critical separations, nocritical alignments and only low voltage drivers. The technology of thepresent invention can be readily scaled up to make extremely largetelevision screens.

OBJECTS OF THE INVENTION

It is therefore a principal object of the present invention to provide atelevision screen that is made from a large plurality of light emittingdiode slivers manufactured in accordance with a unique low cost methodwhich obviates the prior art requirements for critical separationdistances and alignments and the use of high voltage drivers.

It is an additional object of the present invention to provide a uniqueflat screen display for use in televisions and the like and which may bereadily employed for manufacturing large screen televisions that areboth technically feasible and relatively inexpensive.

It is still an additional object of the present invention to provide amethod for manufacturing large screens for televisions and the like,wherein a large plurality of light emitting diode devices are disbursedin molten-glass fibers within which they may be readily aligned betweenalignment plates by exploiting the intrinsic dipole moment of such lightemitting diodes and then subsequently applying transparent horizontaland vertical electrodes forming addressable pixels wherever suchelectrodes cross.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the invention, as well asadditional objects and advantages thereof, will be more fully understoodhereinafter as a result of a detailed description of a preferredembodiment when taken in conjunction with the following drawings inwhich:

FIG. 1 is a cross-sectional drawing of a wafer grown in accordance withthe first step of the method of the present invention;

FIG. 2 is a perspective view of the vertically etched wafer inaccordance with a second step of the present invention;

FIG. 3 is a view similar to that of FIG. 2, but showing a selectiveetching in accordance with a third step of the present invention;

FIG. 4 is a representation of a collection of light emitting diodeslivers mixed within a curable optical epoxy in accordance with the nextstep of a first embodiment of the present invention;

FIG. 5 is a representation of the application of the mixture of FIG. 4to the space between a pair of large conducting plates in preparationfor the alignment step of the first embodiment of the present invention;

FIG. 6 is a representation of the alignment step of the firstembodiment;

FIG. 7 is a representation of the curing step of the first embodiment;

FIG. 8 is a representation of a post-epoxy curing etching step of thefirst embodiment;

FIGS. 9 and 9a represent the top view and side view, respectively, of asubsequent step in the present invention wherein electrodes are appliedto one surface thereof;

FIG. 10 illustrates the application of a glass substrate to the surfaceof the present invention to which electrodes have been applied in thestep of FIGS. 9 and 9a;

FIGS. 11 and 11a illustrate the application of electrodes to theremaining side of the present invention;

FIG. 12 illustrates a step in the second embodiment of the inventionwherein the LED slivers are formed into glass fibers;

FIG. 13 illustrates an additional step in the second embodiment whereinindividual fibers of different colors are positioned side by sidebetween two alignment plates; and

FIG. 14(a), (b) and (c) illustrate the alignment step of the secondembodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now first to FIG. 1, it will be seen that the first step inthe process of the present invention comprises the step of growing awafer providing a light emitting PN junction 10 and a "sacrificial"buffer layer 12 of aluminum arsenide (AlAs). Contact layers 14 and 16are grown on both sides of the PN junction and a gold germanium contact18 is alloyed at this stage of the process. The resulting waferconfiguration comprises a gallium arsenide base 20 upon which there isplaced an aluminum arsenide buffer 22, a PN junction 10 of aluminumgallium arsenide separated from the buffer 12 on one side by positivelydoped gallium arsenide 14 and negatively doped aluminum gallium arsenide16 on the other side thereof. The positively and negatively doped layersabove and below the junction constitute the contact layers. Finally, acontact 18 of gold and germanium is alloyed to the upper surface ofwafer.

Referring to FIG. 2, it will be seen that the second step of the presentinvention comprises the use of a vertical etching technique, such asreactive ion etching, to create a plurality of parallel columns 19. Thewafer is etched so that all of the grown layers are uncovered, that is,all of the layers above the substrate. In FIG. 3, it will be seen thatthe third step of the present invention comprises the step of applying achemical etchant, such as hydrofluoric acid, that will selectively etchonly the sacrificial aluminum arsenide, thus breaking off the individuallight emitting diode columns to form a plurality of light emitting diodeslivers 23. As shown in FIG. 4, these light emitting diode slivers arethen mixed with a curable optical epoxy 24 such as Norland Products,Inc., Part No. NOA60 or a monomer such as methylmethacrylate. Typically,such epoxys may be cured either by applying heat, ultra-violet light orone or more chemical constituents. However, before this curing processby any of these well known means is implemented, the combination oflight emitting diode slivers and curable optical epoxy is poured betweentwo large conducting plates 26 and 28 separated by a distance which isslightly greater than the length of the light emitting diode slivers 23.This step in the process of the present invention is illustrated in FIG.5. In typical applications for a preferred embodiment of the inventionas contemplated herein, each light emitting diode sliver would have alength of approximately 20 micrometers and a diameter of approximately 2micrometers. Accordingly, in typical applications of the step of thepresent invention illustrated in FIG. 5, the separation between theconducting plates would be approximately 25 micrometers.

The next step of the present invention which is illustrated in FIG. 6herein, is that of applying an electric field by generating a voltageacross the plates between which the mixture of light emitting diodeslivers and curable optical epoxy has been placed. This electric fieldacts to identically align the light emitting diode slivers because PNjunctions have an intrinsic dipole moment. It is believed that theamount of voltage for the space contemplated between the conductingplates required to align the light emitting diode slivers, would be onthe order of 10 volts and the amount of time required to permit thealignment of the light emitting diode slivers would be on the order of10 seconds or less, depending, of course, on various parametersassociated with the dimensions and materials such as the viscosity ofthe uncured optical epoxy.

The next step in the process of the present invention is that of curingthe epoxy. Such curing will, of course, depend upon the nature of theepoxy. FIG. 7 illustrates that this step is accomplished by theapplication of heat to the plates which form the upper and lowerboundaries of the light emitting diode sliver/epoxy mixture. However,this step may also be carried out by the application of ultra-violetlight or the use of a chemical additive which initiates the curing ofthe epoxy. Thus, the step of the present invention illustrated in FIG. 7should be understood as a step which does not necessarily require curingby the application of heat, but is instead deemed to be a generic curingstep, the details of which depend upon the nature of the epoxy materialused therein.

The next step of the present invention, which is illustrated in FIG. 8,comprises the step of removing one of the plates used previously toapply an electric field across the mixture of epoxy and LED slivers.After the plate has been removed, the epoxy immediately beneath theplate is chemically etched to uncover all gold germanium alloyed ends ofthe aligned light emitting diode slivers as shown in FIG. 8. The nextstep in the process of the present invention is the application ofelectrodes 30 to the exposed and etched surface of the display, that is,to one axial surface of the LED slivers using standard transparentelectrodes and lithographic techniques. This step is illustrated inFIGS. 9 and 9a. These electrodes define the height of the pixel and inthe preferred embodiment of the invention shown herein, areapproximately 1 millimeter in width. These electrodes are then attachedto a sturdy substrate 32, such as glass, with additional epoxy 34 asshown in FIG. 10. Next, the other conducting plate is removed in thesame manner. The epoxy is etched and another set of electrodes 35 areapplied to that surface, in the same manner as previously described.However, in this particular instance, the electrodes are appliedorthogonally to the previously applied electrodes as shown in FIG. 11and 11a. This step completes the process.

The manufactured product of the present invention may be described as alarge array of light emitting diodes whose pixels are easily addressablewith low voltage drivers connected to horizontal and vertical lines. Thebrightness of the light emitting diodes is diluted to current TV lightoutput levels and power consumption is spread over a large area. Mostimportantly, the manufacturing process of the present invention isexceedingly simple, involving no handling of individual pixel elementsand no critical alignments over the area of the screen. The pixels areformed wherever horizontal and vertical electrodes cross incross-section. The area of each pixel is expected to be approximatelyone square millimeter, while the sliver density is expected to beapproximately one hundred per square millimeter. This obviates the needfor any special processing equipment for aligning the light emittingdiode slivers with the electrodes. It is only necessary to produce astatistical average of about 100 light emitting diode slivers per pixel.By controlling the sliver density within the epoxy when the mixture ismade, the brightness of the screen can be controlled. In addition,because the screen is solid and does not rely on any criticalseparations, large screens can be built without incurring any scalingproblems.

Because the light emitting diode slivers are expected to be relativelysparse within the clear epoxy and because the entire structure isreasonably thin, color screens can be made by manufacturing three suchscreens, using red, green and blue light emitting diode wafers. Thesescreens can then be stacked, one on top of another. The light of thescreen furthest back, is reduced slightly as it passes through themostly transparent upper screens. By controlling the electrodes withred, green and blue information, color displays can be readily achieved.

Light emitting diode material outside the depletion region of the PNjunction acts as an absorber for the light produced. Because the lightemitting diodes are aligned to point at the viewer, the light directedtoward the viewer will be mostly absorbed. Light emitted out from thesides will enter the epoxy and some will become scattered toward theviewer. A simple way to alleviate this absorption is to use doubleheterojunction light emitting diodes which are optically transparent tothe emitted light. In this manner, the viewer will receive directemitted diode light.

A second embodiment of the invention comprises a different method ofmixing and placing the LED slivers between alignment plates. In thissecond embodiment, instead of using a curable epoxy material, glass isused because it is amenable to vacuum deposition of electrodes, has avery controllable etch rate and has a high degree of resistance to heatand moisture. A low melting temperature glass is used to avoid damagingthe LED slivers with excessive heat. In the first step of thisalternative embodiment, that is the step performed first after the LEDslivers 23 are fabricated as previously described, the LED slivers aremixed with molten glass 40 using heater coils 46 and a tension feedbackmechanism 48 and formed into a cylinder as shown in FIG. 12. The LED andmolten glass mixture is drawn into a fiber 44. The tension feedbackmechanism 48 shown in FIG. 12 monitors the diameter of the fiber 44 andadjusts the tension to the diameter at a desired constant dimension.Three different color fibers are made, one for each of the colors red,green and blue. The fibers receive plastic coating 49, are wound on aspool 50 and then sliced or cut into lengths equal to the screen widthand placed between two alignment plates 52 as shown in FIG. 13. Theplates are then heated and a voltage is applied to align the LED slivers23 as shown sequentially in FIGS. 14(a), (b) and (c). The glass fibers44 flow together to form a continuous plate 54 of material which canthen be processed as previously described for the first embodiment ofthe invention.

This second embodiment of the invention overcomes certain potentialdisadvantages of the first embodiment. By way of example, certain stepsof the first embodiment are not necessarily amenable to the processingof polymers. The LEDs may react with the plastic material or epoxy oversome length of time and the plastic may become brittle or discolored dueto local heating affects. Achieving uniform etch rates with plastics isdifficult due to the nature of their long polymer chain and amorphousstructure. Furthermore, the deposition of electrodes onto plastic wouldalso be difficult, as this step typically requires the sample to be keptin a vacuum at elevated temperatures. Also, spreading the mixture of LEDslivers and liquid support material over a large screen presents certainproblems involving quality control because only after the screen isbuilt can the uniformity of the LED slivers be tested. Finally, aspreviously described, multi-color displays would be made of multipletransparent screens made of red, green and blue LEDs sandwichedtogether. This technique would, of course, triple the cost as comparedto a monochrome display. On the other hand, this described secondembodiment of the invention allows the LED uniformity to be checked atthe single row level. The fiber spools can be scanned for areas wherethe LED density is poor, using an absorption measurement system. Thisallows bad fiber to be detected and replaced before an entire screen isbuilt. In addition, all three colors are contained in a single sheet ofglass. Thus opaque electrodes can be used for one side of the screen.This permits higher current to be carried and provides a reflective backcontact that will increase directionality. Thus, this second embodimentof the invention introduces desirable quality control in fabricationsteps that can reduce costs substantially while improving quality.Furthermore, this improvement is based upon the use of glass and fibermanufacturing practices, both of which are well known in the industry.

It will now be understood that what has been disclosed herein comprisesa novel display screen structure and method of manufacturing suchscreens for use, for example, in large screen television displays. Theprocess of the present invention is one which can be accomplished withno new materials, no critical geometric requirements such as criticalseparations, and alignments and only low voltage drivers. Thecombination of these features results in a technology which can beeasily scaled to large sizes to provide relatively low-cost largescreens for televisions. An important step in the disclosed embodimentof the present invention is the alignment of a large plurality ofcolumnar-shaped light emitting diode slivers in either uncured opticalepoxy or in molten glass fibers by applying an electric field through amixture of such slivers and epoxy or glass and then curing the epoxy orallowing the glass to harden to effectively fix the light emitting diodeslivers in that aligned configuration. The light being emitted by suchdiodes, is thereafter controlled by orthogonally directed electrodeswhich are optically transparent and which are placed on one or both ofthe opposing surfaces of the thin plate-like structure fabricated inaccordance with the invention.

Those having skill in the art to which the present invention pertains,will now as a result of the applicants' teaching herein, perceivevarious modifications and additions which may be made to the invention.Thus for example, the precise materials disclosed herein for use in thefabrication of the present invention, as well as the steps disclosedherein and the sequence of such steps as disclosed herein, all may bereadily altered while preserving the principal advantageous features ofthe novel screen display of the present invention. Furthermore,additional steps may be added and additional features may be added tothe structure of the result of the process of the present inventionwhile still achieving the novel and highly advantageous characteristicsthereof. Further, while the display screens disclosed herein may be mostadvantageously used in television applications, they are clearly notlimited to such use. Accordingly, all such modifications and additionsare deemed to be within the scope of the invention, which is to belimited only by the claims appended hereto.

We claim:
 1. A method of fabricating an electronic display screen; themethod comprising the following steps:a) providing a light emitting PNjunction wafer on a substrate; b) etching said wafer to form a pluralityof PN junction columns on said substrate; c) separating said columnsfrom said substrate; d) mixing said columns with a molten glass; e)forming a plurality of elongated glass fibers from said mixture ofcolumns and glass; f) aligning said columns in said fibers to form asubstantially uniform array of parallel columns therein; and g) affixingorthogonally oriented electrodes to opposite common axial ends of saidcolumns for applying selected voltages to selected ones of said columns.2. The method recited in claim 1 wherein said wafer comprises asacrificial buffer layer between said junction and said substrate andwherein step c) is performed by etching said sacrificial buffer layer.3. The method recited in claim 1 wherein step f) is performed byapplying an electric field to the fibers of step e).
 4. The methodrecited in claim 1 further comprising the step of:h) affixing theelectrodes on common axial ends of said columns to a glass substrate. 5.A method of fabricating a display screen for generating visual imagesfrom an array of pixels controlled by selectively applied voltages; themethod comprising steps of:a) forming a plurality of light emittingdiode slivers; b) mixing said slivers with molten glass; c) drawing andsliver and glass mixture into a glass fiber and cutting the fiber into aplurality of equal length fibers; d) applying an aligning electric fieldto align said slivers in said fiber; e) exposing the axial ends of saidslivers; and f) affixing transparent electrodes to said axial ends forapplying said voltages to selected ones of said pixels.
 6. The methodrecited in claim 5 wherein in step f) the electrodes affixed to coplanaraxial ends of said slivers are parallel to one another and areperpendicular to the electrodes affixed to the opposite axial ends ofsaid slivers.